1. Field of the Invention
The present invention relates to an organic light emitting device (OLED) and a color display apparatus using the same, and more particularly, to a top-emitting or bottom-emitting OLED, which improves a color gamut and reduces a variation of color with a viewing angle, and a color display apparatus employing such improved OLED.
2. Description of the Related Art
In an organic light emitting device (OLED), holes from an anode combine with electrons from a cathode in an organic emission layer (EML) formed between the anode and the cathode to emit light, thereby forming an image. Since the OLED has excellent display characteristics, such as a wide viewing angle, a faster response speed, a smaller thickness, a lower fabrication cost, and a higher contrast, the OLED has attracted much attention as an advanced flat panel display device.
In general, the OLED may have a multilayered structure in order to improve luminous efficiency. For example, a hole injection layer (HIL) and a hole transport layer (HTL) may be further formed between an anode and an organic EML, and an electron injection layer (EIL) and an electron transport layer (ETL) may be further formed between a cathode and the organic EML. Also, other additional layers may be further formed.
The OLED may emit light in a desired color by forming the organic EML using an appropriate material. Based on this principle, it is possible to embody a color display apparatus using the OLED. For example, in a color display apparatus using an OLED, each of pixels may include a sub-pixel having a red (R) organic EML, a sub-pixel having a green (G) organic EML, and a sub-pixel having a blue (B) organic EML. When different organic EMLs are formed in respective sub-pixels, however, it is difficult to embody large-area high-resolution display apparatuses due to a complicated fabrication process.
In order to overcome the drawback, a white OLED has been proposed. The white OLED may be embodied by forming a plurality of organic emission materials for emitting light in R, G, and B colors in an organic EML or forming two or more pairs of organic emission materials for emitting light in complementary colors. The white OLED creates colors using color filters. In this case, since all sub-pixels include organic EMLs with the same structure, fabricating large-area high-resolution display apparatuses is relatively easy.
Meanwhile, OLEDs may be classified into bottom-emission OLEDs and top-emission OLEDs depending on a direction in which light is emitted from an organic EML. In a bottom-emission OLED, light is extracted to a bottom surface of the OLED on which a thin film transistor (TFT) for driving the OLED is disposed. In a top-emission OLED, a reflective electrode is disposed under an organic EML so that light is extracted to an upper portion of a TFT. Typically, since the top-emission OLED may increase an emission area (or aperture ratio) more than the bottom-emission OLED, the top-emission OLED is more suited for high-resolution OLEDs.
In the top-emission OLED, however, resonant cavities are formed between a reflective electrode disposed under an organic EML and a semi-transmissive electrode disposed on the organic EML. Since resonance caused in the resonant cavities narrows the spectrum of externally emitted light, it is advantageous at extracting only light with a specific wavelength, but it is disadvantageous at extracting white light. Also, a top-emission white OLED using color filters narrows a color gamut and increases a variation of color with a viewing angle.
In order to solve this problem, an OLED for extracting only light with a specific wavelength using a single resonator mode has been proposed. In this case, however, the optical thickness of resonant cavities should be varied according to the wavelength of light. Therefore, fabrication of a color display apparatus using the OLED may involve a very complicated process so that the optical thickness of the resonant cavities may be controlled to be different according to respective R, G, and B sub-pixels.